Thermal conductivity of polyurethane foam is determined by the sum of thermal conductivity (λm) of a polyurethane resin itself, thermal conductivity (λg) of a blowing agent gas component present in a polyurethane foam cell, and radiant thermal conductivity (λr). Among these, the thermal conductivity (λg) of the blowing agent gas component is about 60 to about 70% of the overall thermal conductivity of the polyurethane resin. However, since the use of conventional fluorocarbon-based blowing agents is restricted due to their high global warming potential (GWP) and ozone destruction index, the fluorocarbon-based blowing agents have been replaced by non-halogen hydrocarbon blowing agents, which can cause deterioration in insulating properties due to large cell size and higher thermal conductivity than the fluorocarbon-based blowing agents. In other words, as the gaseous blowing agents having thermal conductivity (λg) are replaced by the non-halogen hydrocarbon blowing agents due to environmental regulations, it is difficult to improve the thermal conductivity, and the thermal conductivity (λm) of the polyurethane resin can be improved only by replacing the urethane foam. Thus, studies have been actively carried out to reduce the radiant thermal conductivity.
Korean Patent Publication No. 2005-73500A discloses a method of improving heat insulating properties of urethane foam, in which a perfluorinated alkene-based nucleating agent is used to decrease the cell size of the urethane foam. However, this method cannot suitably solve environmental problems caused by the perfluorinated alkene-based nucleating agent.
U.S. Patent Publication Nos. 2005/79352 and 2004/176486 disclose urethane foam which contains thermally expandable fine particles. These inventions were developed to replace a flammable blowing agent with the thermally expandable fine particles and could not improve heat insulating properties since the urethane foam contains an excess of the thermally expandable fine particles as compared with the flammable blowing agent.
Meanwhile, U.S. Pat. No. 5,604,265 discloses polyurethane foam which contains carbon black in order to improve heat insulating properties. However, the polyurethane foam may exhibit only limited improvement of heat insulating properties.